I am a beginner level D-SLR user starting off my photography journey with the Canon EOS 1100D. My purchase came with a 18-55mm kit lense and I purchased the 75-300mm marco lense in addition. I bought the camera primarily because I want to capture some good quality photos of my soon-to-arrive son i.e. portrait shots. I have read lots of advice on purchasing the 50mm Canon lense as I know it achieves a lower F/Stop than the lenses I already have thus producing better portrait shots. I know it is also recommended by Gordan Lang on his DSLR Tips site. However, based on the lenses I already have, is it worth me purchasing this additional lense for a better quality portrait shot? Gordon's review says that mounted on a cropped-frame model, the 50mm lense becomes equivalent to an 80mm, which I assume I could acheive with my marco lense? Although I'm not sure what a 'cropped-frame model' is!

First of all, regarding the mathematical part, the crop factor of 1.6 that makes the 50mm lens nearly equivalent to 80mm on a "full frame" camera also applies to your 70-300mm lens, making it equivalent to the 112mm to 480mm focal length range.

Portrait photographers using "full frame" cameras often prefer 85mm lenses. Those of us with the smaller-sensor cameras, with the 1.6 crop factor, may therefore like a 50mm lens for head-and-shoulder portraits at close range, such as inside a room that is not very large. The problem with a 50mm lens is that if one wants an image of mother and baby, the photographer will have to be positioned farther away to get the mother and baby within the frame, which might be difficult inside a small room. A 35mm lens might be the better option. Your kit lens has 35mm and 50mm settings within its zoom range, so it is rather simple to test these focal lengths at the distances one anticipates shooting.

This does not adress all of your questions, but my time is limited at the moment. Probably, another member will be able to provide advice shortly.

To follow on from RexGig, whether a 50mm lens is worth it depends on what you wish to achieve.

You don't get better quality photos per se from having a lower f/stop (the quality of the glass and optics do that) but the benefit of a lower f/stop is that you can achieve a lower depth of field i.e. it is much easier to blur the background whilst keeping the subject in focus, both literally in terms of optics and in terms of your attention when you look at the photo.

If that is the effect you desire, it's very much worth considering getting a 50mm lens. The f/1.8 model is fantastic value for money but you should bear in mind that while it is pretty competent at f/1.8, its best sharpness is achieved at f/2.8 or more though even at f/2.8 you've got a much shallower depth of field than you can possibly achieve with the 18-55mm kit lens in the same surroundings. However, as he said, you'd need to determine if 50mm would be too tight, in which case the 35mm f/2 or 28mm f/1.8 may be better options though both cost considerably more money.

Adding to RexGig's comments about the crop factor, a "cropped-frame model" refers to the size of the sensor. It basically means that, in terms of coverage, the sensor's surface area is "cropped" from the surface area of a 35mm "full-frame" DSLR sensor, which in turn is based on the 35mm film format. If you check the link below, you'll see how it works in practice when you compare an image taken from a full-frame body to an image taken from a 1.6x crop factor body where both are taken at the same position and distance from the subject and at the same focal length.

I would suggest you put your kit lens on 50 and try taking various shots and see if that fits in you style, try indoors and outdoors.

Personally I would prefer the 30mm or 85 if you do more portrait type stuff.

Main reason for getting a prime lens is better image quality ( in particular when stepped down to 2.8 ), large aperture for low light and depth of field.I have the 50 1.8 and I don`t use it much, its build quality and focus speed and accuracy is poor at best. I should have bought the 50 1.4, but in general I prefer the focal length of around 30. I can recommend the Sigma 30 1.4.

My suggestion here would be that you borrowed the lens either from a friend or a camera shop. You can get to test it out and see if it suits you and if you like it. If you don't like it or it can't suit your needs, you could go for a 35mm as suggested by Rorschach. As a last option, you could go for the EF-S 17-55mm f/2.8 IS USM lens, but as a disadvantage, if you ever upgrade to a full-sensor camera, this lens won't be compatible.